Organocatalytic Enantioselective Vinylogous Aldol Reaction of Allyl Aryl Ketones to Activated Acyclic Ketones

Article abstract of DOI:10.1021/acs.orglett.5b03412

The first catalytic asymmetric vinylogous aldol reaction of activated allyls to activated acyclic ketones is disclosed. A variety of activated acyclic ketones, such as trifluoromethyl ketones, α-ketoesters, and α-keto phosphonates, were found to be involved forming diverse γ-selective aldol adducts with high enantioselectivities (up to >99% ee). The method provides an effective, general strategy to access valuable chiral electron-withdrawing group-substituted tertiary hydroxyl-based carboxylic acids.

Full text of DOI:10.1021/acs.orglett.5b03412

Letter
pubs.acs.org/OrgLett
Organocatalytic Enantioselective Vinylogous Aldol Reaction of Allyl
Aryl Ketones to Activated Acyclic Ketones
Zhenzhong Jing,† Xiangbin Bai,† Wenchao Chen, Gao Zhang, Bo Zhu, and Zhiyong Jiang*
Key Laboratory of Natural Medicine and Immuno-Engineering of Henan Province, Henan University, Jinming Campus, Kaifeng,
Henan 475004, China
S
* Supporting Information
ABSTRACT: The first catalytic asymmetric vinylogous aldol reaction of activated allyls to activated acyclic ketones is disclosed.
A variety of activated acyclic ketones, such as trifluoromethyl ketones, α-ketoesters, and α-keto phosphonates, were found to be
involved forming diverse γ-selective aldol adducts with high enantioselectivities (up to >99% ee). The method provides an
effective, general strategy to access valuable chiral electron-withdrawing group-substituted tertiary hydroxyl-based carboxylic
acids.
substituted 2-isoxazolines (B) as CF3-β-THCAs3 found in
hiral electron-withdrawing-group-functionalized tertiary
almost 27000 molecules have synthetic applications in
veterinary medicine and agrochemicals. Notably, CF3-γ-
Chydroxy-based carboxylic acids (EWG-THCAs) consti-
tute a wide range of key structural motifs in molecules with
important biological activities (Scheme 1). Over 2800
THCAs (e.g., molecule C) and CF3-δ-THCAs (e.g., molecule
D) often exhibit anti-HIV activities.4 THCAs featuring other
compounds with potential bioactivities feature CF3-α-THCA
electron-withdrawing groups, such as carboxylic acid derivatives
as the structural unit,1,2 especially Mosher’s acid1a,2 A, which is
(CA-THCAs, e.g., molecules E−H) and phosphonates (PA-
widely used in asymmetric synthesis. The trifluoromethyl-
THCAs, e.g., molecules I−K), on the α−δ positions of
carboxylic acids are also important frameworks of many
bioactive compounds.5 Despite the existence of key catalytic
Scheme 1. Representative Bioactive Compounds
asymmetric strategies to diverse enantiomerically enriched
EWG-THCAs,6−8 a divergent strategy to simultaneously
furnish chiral CF3-(α−δ)-THCAs, CA-(α−δ)-THCAs, and
PA-(α−δ)-THCAs is lacking. It is thus highly desirable to
devise a practical and efficient method to realize this important
task.
In recent years, asymmetric vinylogous aldol (AVA) reaction
has been demonstrated to be expedient in generating
multifunctional chiral alcohols with an easily modified
unsaturated long carbon skeleton, thereby allowing structurally
complex chiral molecules with divergent synthetic targets.9
Retrosynthetic analyses (Scheme 1) of a general AVA reaction
of activated allyls to activated acyclic ketones, including
trifluoromethyl ketones, γ-ketoesters, and α-keto phosphonates,
reveal the possibility of achieving the divergent synthesis of
these desired chiral EWG-(α−δ)-THCAs.
In 2009, Shibasaki and co-workers described a highly
enantio- and γ-selective aldol reaction of allyl cyanide to simple
acyclic ketones, to pioneer the work of activated allyls in the
Received: November 29, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b03412
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
asymmetric vinylogous reaction.10 Reports of vinylogous
reactions involving activated allyls as nucleophiles have been
described since then.11 Nonetheless, the AVA reaction of
activated allyls to activated acyclic ketones is still unmet, and
two formidable challenges remain. The first is γ-selectivity.
Lower electron density on the γ position of activated allyl-
generating dienolates than the α position often leads to the
poor γ-selectivity.12 For example, asymmetric allylic alkylation
of Morita−Baylis−Hillman carbonates with allyl ketones is
highly α-selective.12c For more sterically hindered cyclic
ketimines, good α-selectivity is found in the asymmetric
Mannich reaction of allyl ketones.12d To discriminate the
reactive enolate site, shielding the α position by introducing a
bulky modifier at the α-position13 or using an additional
catalyst14c is necessary.
Another issue is enantioselectivity and universality. No
example to date has been reported of an asymmetric vinylogous
reaction of activated allyls to trifluoromethyl ketones and α-
keto phosphonates, and only modest enantioselectivity was
obtained for the asymmetric reaction of allyl cyanide with α-
ketoesters.10 Universality-wise, no asymmetric protocol has
been able to be effectively performed on trifluoromethyl
ketones, α-ketoesters, and α-keto phosphonates electrophiles.
Herein, we report the first catalytic asymmetric VA reaction of
activated allyls to activated acyclic ketones. The strategy is
general in that it is applicable to diverse activated acyclic
ketones, including trifluoromethyl ketones, α-ketoesters, and α-
keto phosphonates, thus furnishing the desired divergent
synthesis of chiral EWG-(α−δ)-THCAs.
a
Table 1. Optimization of the Reaction Conditions
temp
(°C)
yield
b
ee
c
d
entry catalyst solvent
t (h)
(%)
(%)
E/Z
1
I
toluene
toluene
toluene
THF
25
45
45
45
45
45
45
45
45
45
45
62
38
38
12
77
>20:1
4:1
2
II
III
II
II
II
II
II
II
II
II
IV
IV
25
17
91
3
25
trace
N.R.
10
N.A.
N.A.
80
N.A.
N.A.
3:1
4
25
5
DCM
25
6
Et2O
25
14
88
>20:1
8:1
7
t-BuPh
t-BuPh
t-BuPh
t-BuPh
t-BuPh
t-BuPh
t-BuPh
25
28
92
8
0
43
95
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
9
−10
−20
−10
−10
−10
42
96
10
11
12
13
44
96
e
e
65
96
87
94
ef
,
91
95
a
Reaction conditions: 1a (0.075 mmol), 2a (0.05 mmol), catalyst
(0.005 mmol), 0.5 mL of solvent. Yield of isolated product.
Determined by HPLC analysis on a chiral stationary phase.
Determined by crude H NMR spectra. 1a:2a = 3:1. 2.0 equiv of
b
c
d
e
f
1
Na3PO4 were used as an additive.
Allyl ketones11b,12c,d,14 are easily prepared and most
commonly used as activated allyls for asymmetric direct
vinylogous reactions.14 To probe the feasibility of the desired
reaction on its reactivity, regioselectivity, and stereoselectivity,
our study was thus initiated with the model reaction between
allyl phenyl ketone 1a and trifluoromethyl acetophenone 2a.
The reaction was first performed in toluene at 25 °C, using 10
mol % of L-leucine-derived tertiary amine−thiourea I as the
catalyst15 (Table 1, entry 1). It was observed that most of 1a
was transformed to the α,β-alkene 4a via proton transfer, and
the desired AVA adduct 3a was obtained in only 12% yield.
Although the reactivity was poor, moderate enantioselectivity
and excellent E-selectivity were observed, indicating the
effectiveness of the bifunctional catalyst. Takemoto’s chiral
1,2-cyclohexanediamine-based tertiary amine−thiourea II16 was
then examined, providing 3a in 17% yield with 91% ee and
modest E-selectivity (entry 2). We replaced the thiourea of II
by the urea, but the corresponding catalyst III promoted 1a to
completely generate 4a (entry 3). Next, the reaction was
performed in different solvents using 10 mol % of II as the
catalyst (entries 4−7). The results revealed that tert-
butylbenzene was the most suitable solvent, affording 3a in
28% yield with 92% ee and an 8:1 E/Z ratio (entry 7). A
variation of the reaction temperature showed that the E/Z ratio
of 3a could be raised to >20:1 at a lower temperature (entries
8−10) and −10 °C was optimal (entry 9). When the amount of
1a was increased to 3.0 equiv, 65% yield of 3a was achieved
(entry 11). The adduct 3a could be obtained in 87% yield and
94% ee by utilizing catalyst IV,16c an analogue of catalyst II with
a pyrrolidine as the tertiary amine moiety (entry 12). The basic
Na3PO4 additive was found to further positively influence both
yield and enantioselectivity (entry 13).
1 to various trifluoromethyl ketones 2 (Scheme 2). The
reactions were often complete within 24−72 h, providing
a
Scheme 2. Direct AVA Reaction between 1 and 2
a
Reaction conditions: 1 (0.3 mmol), 2 (0.1 mmol), catalyst (0.01
b
mmol), Na3PO4 (0.2 mmol), 1.0 mL of tBuPh. After a single
recrystallization, ee > 99%. The ee value and yield were obtained after
c
a single recrystallization. Initial data: 80% yield, 85% ee.
adducts 3a−y with yields of 60−98%, enantioselectivites of 70−
99% ee, and E/Z ratios of more than 20:1. It should be noted
that allyl ketones 1 with the ortho-substituents on phenyl rings
(3r and 3w) gave depressed enantiomeric excesses, whereas
allyl 2-thienyl ketone gave 3y with excellent enantioselectivity.
With the optimal reaction conditions in hand, we
investigated direct AVA reaction of distinct allyl aryl ketones
B
DOI: 10.1021/acs.orglett.5b03412
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
We then selected two classes of acylic activated ketones 5, i.e.
α-ketoesters and α-ketophosphonates for further examination
(Scheme 3). Under slightly modified reaction conditions (10
Scheme 4. Synthesis of (α−δ)-CF3-THCAs
a
Scheme 3. Direct AVA Reaction of 1 to 5
H2SO4, and H2O), ketone 13 as the precursor of insecticides
B3c was obtained with >99% ee. Since the aldehyde group of 9
inhibited the methylation of the tertiary alcohol, an acetal
protecting method by using trimethyl orthoformate was
attempted. Unsurprisingly, yields of both protection and the
corresponding deprotection by TFA were excellent. Following
an efficient acetylation of aldehyde and ozonization, (R)-
Mosher’s acid A was obtained with >99% ee. Clearly, these
strategies could be readily employed to address the divergent
synthesis of chiral CA-(α−δ)-THCAs and PA-(α−δ)-THCAs.
In summary, we have developed the first catalytic asymmetric
AVA reaction of activated allyls to activated acyclic ketones. A
series of activated acyclic ketones, such as trifluoromethyl
ketones, γ-ketoesters, and α-keto phosphonates, were suitable
in the established strategy, thus generating diverse γ-selective
aldol adducts with high enantioselectivities (up to >99% ee).
The reported synthetic method provides an expedient approach
to various biologically important chiral EWG-substituted
tertiary hydroxy-based α-, β-, γ-, and δ-carboxylic acids through
divergent synthesis with excellent results. Further investigations
into the employment of activated allyls in unprecedented AVA
reactions, and the resulting divergent synthesis of significant
bioactive molecules, are ongoing and will be reported in due
course.
a
Reaction conditions: 1 (0.3 mmol), 5 (0.1 mmol), catalyst IV (0.01
mmol), K2HPO4 (0.2 mmol), 0.8 mL t-BuPh, −20 °C.
mol % of catalyst IV, 2.0 equiv of K2HPO4, −20 °C), a range of
α-ketoesters were first explored, including aromatic substrates
with diverse substitution patterns and methyl/ethyl α-
ketoisovalerate as the representative of aliphatic substrates.
The corresponding AVA adducts (6a−m) were obtained in
60−92% yields with 89−97% ee and >20:1 E-selectivity within
20−52 h. The different ester groups of α-ketoesters resulted in
similar reactivity and enantioselectivity. α-Ketophosphonate
also proved to be a viable substrate, giving α-hydroxyphosph-
onates 6n−o in excellent enantioselectivity.
To demonstrate the synthetic value of this work, a series of
transformations from vinylogous aldol adducts were then
performed (Scheme 4). The δ-CF3-THCA derivative 7 was
obtained in 86% yield with 95% ee from the reduction of 3u
with H2 on Pd/C, followed by a Baeyer−Villiger rearrangement
(BVR) with m-CPBA. Reduction of adduct 3a with H2 on Pd/
C for 6 h afforded CF3-based tertiary alcohol 8 featuring a long
carbon chain and without compromising the ee value. After
ozonolysis, 3a could be conveniently converted to aldehyde 9,
which herein is demonstrated as a common intermediate to
access (α−γ)-CF3-THCAs (Scheme 1). First, through a
sequence of Wittig reaction, hydrolysis, and reduction, alcohol
11 as the γ-CF3-THCA derivative was attainable with >99% ee.
Jones oxidation of 9 presented β-CF3-THCA 12 in 92% yield.
By treatment of PhMgBr and then Jones reagents (CrO3,
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.5b03412.
■
S
General information, general experimental procedures,
procedures of divergent synthesis of THCAs, character-
ization of adducts including HPLC and NMR spectra of
the compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: chmjzy@henu.edu.cn.
C
DOI: 10.1021/acs.orglett.5b03412
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(e) Mikami, K.; Kawakami, Y.; Akiyama, K.; Aikawa, K. J. Am. Chem.
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Author Contributions
†Z.J. and X.B. contributed equally to this work.
Notes
The authors declare no competing financial interest.
́
13, 4020. (j) Crespo-Pena, A.; Monge, D.; Martín-Zamora, E.; Alvarez,
̃
ACKNOWLEDGMENTS
We are grateful for the grants from NSFC (21072044) and
PIRTUHP (14IRTSTHN006).
■
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Org. Lett. XXXX, XXX, XXX−XXX